Process for the removal of ammonia from an ammonia-containing gas stream

ABSTRACT

The invention is directed to a process for the removal of ammonia from an ammonia-containing gas stream by treating the ammonia in the ammonia-containing gas stream with an acid, during which treatment an aqueous stream comprising an ammonium salt, wherein the aqueous stream comprising the ammonium salt is treated with electrodialysis, whereby the acid is recovered and an aqueous stream comprising an ammonium hydroxide salt is formed.

This application is the US national phase of international applicationPCT/EP2005/012199 filed 11 Nov. 2005 which designated the U.S. andclaims benefit of EP 04078338.3 and EP 05075478.7, dated 8 Dec. 2004 andFeb. 28, 2005, respectively, the entire content of which is herebyincorporated by reference.

FIELD OF THE INVENTION

The invention is directed to a process for the removal of ammonia froman ammonia-containing gas stream by treating the ammonia in theammonia-containing gas stream with an acid, during which treatment anaqueous stream comprising an ammonium salt is formed.

BACKGROUND AND SUMMARY

Ammonia has to be removed from gas streams that are vented into the air,because it causes environmental problems. Governmental regulations withrespect to ammonia emissions will become stricter in future.

A process for the removal of ammonia from an ammonia containing gasstream is described, for instance, in U.S. Pat. No. 4,424,072. In thispatent specification is described that the ammonia is removed from thegas stream exiting from the top of a prilling tower in a urea plant bycontacting the gas stream in a scrubber with a non-volatile, dilute acidsolution whereby the NH₃ is absorbed. According to U.S. Pat. No.4,424,072, non-volatile acid solutions include inorganic acids such asphosphoric acid, sulphuric acid and nitric acid as well as organic acidssuch as citric acid, oxalic acid and comparable non-volatile organicacids. According to U.S. Pat. No. 4,424,072 the ammonia-free gas streamis vented to the air. It is mentioned that the obtained aqueous streamcomprising the ammonium salt leaving the scrubbing section in theprilling tower can be recycled to the urea production process.

The drawback of using the process according to U.S. Pat. No. 4,424,072,wherein the aqueous stream comprising the ammonium salt is recycled tothe urea production process, is that contaminations by ammonium saltsmay occur in the final product, in this case urea, which contaminationsare undesirable. For instance, ammonium salts in urea will usuallyrender this urea unsuitable for the preparation of melamine. Theseparate processing of these ammonium salts (e.g. as a by-product) costsmoney and energy or often poses an environmental problem.

It has been found that the aforementioned drawback can be eliminatedwith a process wherein the aqueous stream comprising the ammonium saltis treated with electrodialysis, whereby the acid is recovered and anaqueous stream comprising an ammonium hydroxide salt is formed.

By performing the electrodialysis on the aqueous stream comprising theammonium salt the acid that has been used to remove the ammonia can berecovered and an aqueous stream comprising the ammonium salt is notrecycled to the urea production causing contamination of the producedurea.

According to the process for the removal of ammonia from anammonia-containing gas stream according to the invention the aqueousstream comprising the ammonium salt is treated with electrodialysis.

Here and hereafter electrodialysis is defined as an electrolyticalprocess comprising an anode and a cathode which also comprises at leastone membrane situated between the anode and cathode. This membrane canbe an anion-permeable membrane, a cation-permeable membrane or acombination of one or more of these membranes with at least two bipolarmembranes. Anion-permeable membranes and cation-permeable membranes arepermeable for anions respectively cations when these ions are attractedby the cathode respectively the anode. Bipolar membranes consist of ananion-permeable membrane and a cation-permeable membrane laminatedtogether. When this bipolar membrane is oriented such that thecation-permeable membrane faces the cathode water is split into protonsand hydroxyl ions.

During the treatment of the aqueous stream comprising the ammonium saltthe acid used for converting the ammonia in the ammonia-containing gasstream is recovered. This acid can thus be reused for converting theammonia in the ammonia-containing gas stream. Acids that can be used forconverting the ammonia are, for example, the organic acids and inorganicacids mentioned in U.S. Pat. No. 4,424,072, as described above.

During the process according to the invention also an aqueous streamcontaining ammonium hydroxide is formed during the electrodialysis ofthe aqueous stream comprising the ammonium salt. After its formationthis aqueous stream containing ammonium hydroxide can be heat treated,whereby a gaseous ammonia stream is formed.

The ammonia-containing gas stream can originate from various chemicalprocesses, like the ammonia, urea and melamine production processes, butalso from agricultural sources. The process according to the inventionis in particular suitable for the treatment of ammonia-containing gasstreams that contain low amounts of ammonia. These ammonia-containinggas streams are difficult to treat in an other way, for instance byseparation and condensation, for removal of ammonia from these gasstreams. Examples of ammonia-containing gas streams containing lowamounts of ammonia are the gas streams that leave the prilling or thegranulation sections of urea plants.

When the ammonia-containing gas stream, to be treated by the processaccording to the invention, is originating from the ammonia, urea or themelamine production processes the gaseous ammonia stream can be recycledto these processes. Before recycling the gaseous ammonia stream thegaseous stream can be treated to concentrate the ammonia in the gaseousstream.

Electrodialysis is performed in an electrochemical cell comprising ananode and cathode separated by an anion-permeable membrane or acation-permeable membrane. Also a combination of one or more of thesemembranes with at least two bipolar membranes can be used. For theconversion of the ammonium salt in the aqueous stream in such a way thatan acid is recovered and an ammonium hydroxide salt is formed,preferably the anode and the cathode are separated by at least oneanion-permeable membrane. More preferably the electrochemical cell alsocomprises bipolar membranes and a cation-permeable membrane.

The invention is also directed to an electrodialysis section fortreatment of an ammonia-containing gas stream comprising

-   -   a scrubber, wherein an ammonia-containing gas stream is        contacted with an acid,    -   an electrodialysis apparatus, wherein a stream comprising an        acid and a stream comprising an ammonium hydroxide salt are        generated.        Preferably, the electrodialysis section also comprises a        stripper wherein the stream comprising an ammonium hydroxide        salt is heated and a gaseous ammonia stream is formed.

The electrodialysis section preferably is incorporated into a ureaplant, comprising a prilling section or a granulation section, to treatthe ammonia-containing gas stream leaving the prilling or granulationsection.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained in detail with reference to theaccompanying figures, wherein

FIG. 1 is a schematic diagram of an electrodialysis section according toan embodiment of the invention; and

FIG. 2 is a schematic diagram of a three-compartment electrochemicalcell that may be employed in the electrodialysis section.

DETAILED DESCRIPTION

The embodiment of an electrodialysis section according to an embodimentof the invention as depicted in FIG. 1 comprises a scrubber (SC) whereinan ammonia-containing gas stream (1) is contacted with an acid solution(3). Clean air is vented from the scrubber via (2). The acid solutioncontaining also the captured ammonia as an ammonium salt is transportedwith a pump partly to the electrodialysis apparatus (ED) and partly backto the scrubber. The electrodialysis apparatus comprises at least onemembrane located between an anode and a cathode. A stream comprising theacid (4) is generated in the electrodialysis apparatus (ED) and recycledto the scrubber (SC) and an other stream comprising an ammoniumhydroxide salt (5), also generated in the electrodialysis apparatus, isfed to the stripper (ST). In the stripper (ST) the ammonium hydroxide isconverted with the aid of steam (6) in a gaseous ammonia stream (7),comprising water.

Two examples of possible configurations of an electrodialysis apparatusaccording to the invention will be explained in more detail.

According to the first configuration the electrodialysis apparatusconsists of electrochemical cells comprising one anion-permeablemembrane situated between an anode and a cathode dividing the cell in ananode compartment and a cathode compartment. To the cathode compartmentthe solution comprising the ammonium salt is fed. In the electrodialysisapparatus the acid is formed in the anode compartment and the ammoniumhydroxide salt is formed in the cathode compartment. A sulfuric acidsolution with a pH between 0 and 2 can be used as the acid. In thescrubber ammoniumsulphate is formed. A part of the solution leaving thescrubber and containing the ammoniumsulphate is fed to the cathodecompartment of the electrochemical cell. The sulphate ions aretransferred through the membrane and are thereafter, in the anodecompartment, converted into sulfuric acid. The ammonium ions are, in thecathode compartment, converted into ammonium hydroxide. Part of thesulfuric acid solution is recycled to the scrubber and part of theammonium hydroxide solution is treated in the stripper.

According to the second configuration the electrodialysis apparatusconsists of electrochemical cells comprising an anion permeable membraneand/or a cation permeable membrane and bipolar membranes situatedbetween an anode and a cathode and ordered in a way known to a personskilled in the art. The electrochemical cell can for instance be aso-called three-compartment cell according to FIG. 2.

The ammonia-containing gas stream was contacted with a nitric acidsolution with a pH between 0 and 2 in the scrubber. A part of theresulting solution comprising ammoniumnitrate is fed to theelectrochemical cells in the electrodialysis apparatus. Theelectrochemical cell is composed of an anion-permeable membrane (A), acation-permeable membrane (C) and bipolar membranes (B) in anarrangement according to FIG. 2. The solution comprising ammoniumnitrate(S) was fed to the electrochemical cell. Water was split within thebipolar membrane and a nitric acid solution (Y) and ammonium hydroxidesolution (X) are formed. Part of the nitric acid solution is recycled tothe scrubber and part of the ammonium hydroxide solution is treated inthe stripper.

The invention claimed is:
 1. A process for the removal of ammonia froman ammonia-containing gas stream which comprises: (a) treating ammoniain an ammonia-containing gas stream with an acid to form an aqueousstream comprising an ammonium salt, (b) treating the aqueous streamcomprising the ammonium salt with electrodialysis to thereby recover theacid and form an aqueous stream consisting of ammonium hydroxide, (c)heat treating the aqueous stream consisting of ammonium hydroxide tothereby form a gaseous ammonia stream, and (d) recycling the gaseousammonia stream to a process for the production of urea, wherein theammonia-containing gas stream is an off-gas stream from a prilling orgranulation section of a process for the production of urea.
 2. Theprocess according to claim 1, which comprises performing theelectrodialysis in an electrochemical cell comprising an anode andcathode separated by an anion-permeable membrane, a cation-permeablemembrane or a combination of one or more of these membranes with atleast two bipolar membranes.
 3. The process according to claim 1, whichcomprises performing the electrodialysis in an electrochemical cellcomprising an anode and cathode separated by an anion-permeablemembrane.
 4. The process according to claim 1, which comprisesperforming the electrodialysis in an electrochemical cell comprising ananode and cathode separated by an anion-permeable membrane, acation-permeable membrane and bipolar membranes.